Twin screw plastic extruders typically comprise a first cylindrical barrel having a hollow bore extending through the barrel from end to end. A typical cross-section of such hollow bore approximates the shape of a “figure 8” resulting from a longitudinal intersection of paired channels, each channel having a circular cross-section. Paired counter-rotating screw impellers or helical auger impellers are rotatably mounted within such channels, the outer peripheries of the helical flights of such screws being closely fitted to the inner dimensions of such channels. Typically, the flights of such screws intermesh along the longitudinal intersection of the circular channels. Counter-rotation of the screws within such channels effectively conveys plastic powders and melts along the channels, and simultaneously mixes plastic powders and melts at the intersection of the flights.
Precise temperature control of plastic powders and melts conveyed and mixed within the bore of a plastic extruder barrel is typically provided via electric resistance heating and oil flow heating. Electrical resistance heaters are typically wrapped around the outer peripheral surface of the barrel of a twin screw plastic extruder, such heaters effectively controlling the temperature of the barrel. Typically, the roots of the twin screws of the plastic extruder include a hollow bores which serve as oil flow conduits, the bores typically having oil injection tubes extending axially therethrough. Heated oil (or cooling oil) flowing forwardly through such oil injection tubes and returning rearwardly through the oil conveying bores of the roots of such screws provides precise temperature control of the screws. By precisely controlling the temperature of the screws and the temperature of the barrel, precise control of the temperatures of plastic powders and melts which are mixed within and conveyed through the first barrel of the extruder is achieved.
Plastic melt which emits from the downstream output end of the above described “figure 8” shaped extruder barrel bore is necessarily conveyed to an extrusion die for final forming into useful extruded plastic products. An extrusion die adapter is typically utilized to achieve such conveyance, such adapter typically comprising a second axially mounted barrel section whose hollow bore has a “figure 8” shaped input end closely matching the cross-sectional shape of the bore of the first barrel, and whose output end comprises a single smaller channel, typically having a circular cross-sectional shape. The walls of the hollow bore of such extrusion die adapter necessarily define a melt flow conveying channel extending between the “figure 8” shaped input end and the smaller circular channel output end. The wall of the upstream end of such bore typically defines a pair of laterally intersecting conical concavities which taper inwardly and forwardly toward the output end of the adapter.
Heated plastic melt flow within such intersecting conical concavities is commonly irregular, resulting in stagnation of melt on or near the surface of such concavities at or near their input ends. Such melt stagnation undesirably results in hardening and adhesion of small portions of plastic melt upon the interior walls of the adapter. Such hardening and adhesion progressively results in degradation of the hardened plastic which undesirably tends to release into plastic melt which flows downstream into extrusion dyes, resulting in undesirable discoloration and degradation of the final plastic extrusion product.
The instant inventive plastic extruder screw tip solves the above described problem by providing a plurality of cantilevered or laterally extending dashers which move rotatably through the plastic melt as the melt travels through the extrusion dye adapter, such dashers passing through the above described areas which are prone to plastic melt stagnation. The rotary movement of the dashers within the extrusion dye adapter prevents plastic stagnation from occurring, and prevents undesirable discoloration and degradation of plastic extrusion products.